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Heredity/Transcript
Transcript Text reads: The Mysteries of Life with Tim and Moby Moby waters plants in a garden. Moby walks over to a smaller plant. Tim observes him. MOBY: Beep. A boy, Tim, is holding a sheet of paper. TIM: Dear Tim and Moby. Tim reads from a typed letter. TIM: My parents both have brown eyes, but mine are blue. What's going on? From, Tyra. Hmm, I bet there are blue-eyed people somewhere in your family tree. Eye color is just one of many physical traits we inherit from our parents. That passing of traits from parents to offspring is known as heredity. An image shows Tim and his parents on their family tree. TIM: But remember that parents inherit traits from their parents, and so on as far back as your family tree can go. The view of Tim's family tree expands to show his fraternal and maternal grandparents. Then it expands to show more of the family. TIM: Let's look at some pea plants to see how it all works. MOBY: Beep? TIM: Yes, pea plants. Every living thing inherits traits. Back in the 1800s, a monk named Gregor Mendel used pea plants to study how traits like height, color, and shape are inherited. An image shows Gregor Mendel and pea plants. TIM: When Mendel crossed short plants with tall plants, he noticed that only tall plants were produced. Whatever produced these short plants seemed to disappear. An animation shows a series of tall plants decending from tall and short parent plants on a family. TIM: Mendel called that tall height trait the dominant factor because it dominated, or covered up, the short height form. He called the short height trait the recessive factor. By the second generation of pea plants, those recessive short plant forms cropped up again. After tons of research, Mendel was able to predict that, on average, one out of every four pea plants would be short. The plant family tree animation shows three tall plants and one short plant produced by two tall plants. TIM: Each plant has two genes that determine height. Moby holds up two fingers. TIM: The tall gene, represented here by a capital T, is dominant, which means it only takes one of these genes to make a plant tall. It takes two short genes, represented by a lowercase t, to make a plant short. An image shows two tall plants and a short plant. The two tall plants show one represented by two capital T's, the other by a capital T and a lowercase t. The short plant is represented by two lowercase t's. TIM: In the nineteenth century, a British geneticist named Reginald Punnett also experimented with peas and developed something called the Punnett Square. Among other things, this Punnett square lets you predict the likelihood of what you would get if you crossed a tall plant with a short plant. An offspring gets one gene from each parent, either a tall or a short gene. An animation shows how the tall or short gene of each parent goes into one of the four quarters of the Punnett square. Each quarter receives the same uppercase T and lowercase t. Each of the four quarters represents an offspring. TIM: Each plant produced by these two plants will be tall because each one has one of those dominant tall genes. But what happens if two of these new plants produce offspring? There are lots of ways this could turn out, but the most likely result is that we'd get three tall plants and one short one. The Punnett Square animation shows one quarter with two capital T's, two with an uppercase T and lowercase t, and one with two lowercase t's. The plant in the quarter with two lowercase t's is short, while the other three plants are tall. TIM: Mendel only figured this stuff out after carefully studying 30,000 individual pea plants over an eight-year period. MOBY: Beep? TIM: Well, I guess it's pretty reasonable to assume that he had peas for dinner every night. MOBY: Beep. Beep. TIM: I think you're missing the point. Moby's thought bubble shows him covered up to his neck in peas, while holding up two big bowls of peas. TIM: (voice only) Moby? Moby! Category:BrainPOP Transcripts